In recent years, a water absorbent resin having high water absorbency has been developed and often used primarily in disposable applications as absorbent articles such as paper diapers and sanitary napkins, as well as water retention agents for agricultural and horticultural use, industrial water stopping material, and the like. As for such water absorbent resins, many hydrophilic polymers and monomers as raw materials thereof have been proposed, and among them, polyacrylic acid (salt)-based water absorbent resins that use acrylic acid and/or salt thereof as a monomer have been industrially most frequently used, due to their high water absorption performance. Such a polyacrylic acid (salt)-based water absorbent resin is obtained as a polyacrylate by neutralizing acrylic acid and then polymerizing the product, or by polymerizing acrylic acid and then neutralizing polyacrylic acid thus obtained. Such neutralization and polymerization processes are disclosed in Patent Documents 1 to 4 or Non-Patent Document 1.
Also, since the primary use of the water absorbent resins lies in hygienic materials such as paper diapers and sanitary napkins, particular caution is taken for safety. Especially, since a proportion of water absorbent resins in diapers is increasing recently, water absorbent resins containing a reduced amount of highly reactive components, for example, residual monomers in the water absorbent resins are preferred. On the other hand, because residual monomers such as acrylic acid (salts) have an acid odor, also from the viewpoint of odor of the water absorbent resin, water absorbent resins having reduced amounts of residual monomers are preferred. Regarding the method for producing such a water absorbent resin, various technologies have been proposed, and for example, in order to reduce an amount of residual monomers that are generated in heat-treating a polymer, a method of performing polymerization using an acrylic acid containing a reduced amount of acrylic acid dimer (Patent Document 5), a method of performing polymerization using an aqueous monomer solution with a reduced amount of β-hydroxypropionic acid (Patent document 6), and the like have been known. Furthermore, as methods for reducing an amount of residual monomers by using an additive, a method of adding a nitrogen compound (Patent Documents 7 and 8), particularly a method of performing neutralization with ammonia (Patent Document 6), a method of adding a persulfuric acid salt to a polymer and heating the polymer (Patent Document 9), a method of adding a reducing agent to a water-containing gel and heating the water-containing gel (Patent Document 10), a method of adding a reducing agent, and a surfactant and/or water-insoluble fine particles to a water absorbent resin and heating the mixture (Patent Document 11), a method of adding any one of an inorganic reducing agent, an initiator, an oxidizing agent and a reducing agent to a fine powder and granulating the mixture to form a gel, and heating the gel (Patent Document 12), a method of adding a photoinitiator and then exposing the mixture to sunlight (Patent Document 13), and the like have been known. Also, a method of promoting reduction of an amount of residual monomers in a drying step by using a particular drying method has been known (Patent Document 14).
However, although trace components in acrylic acid or an aqueous monomer solution are controlled, there have been limitations on an amount of residual monomers that can be reduced only by suppressing the residual monomers that newly increase when the polymer is heat-treated. Furthermore, in the case where an additive is used, even if the residual monomers could be reduced, it is disadvantageous in view of cost, and also, there may be a problem with the safety of the additive itself, or there may be occasions where coloration occurs particularly when the polymer is heated, or a foul odor different from that of the residual monomers may be generated. Therefore, none of the products was found to be preferable as a water absorbent resin. Furthermore, due to use of large amounts of additives, there have been problems such as an increase in cost, as well as coloration and generation of foul odor of water absorbent resin, and a decrease in the water absorption performance, which are caused by additives.